A video display operates by displaying video images in response to a composite video signal received from a video source. The composite video signal comprises both luminance (luma) and chrominance (chroma) information. The luma portion contains the brightness, or black-and-white information for a given series of pixels, and the chroma portion contains the color information for the series of pixels. There are several standards for video signals commonly in use today, one being the National Television Standards Committee (NTSC) standard, which is used widely in the United States and Japan. Another standard for video signals is the Phase Alternating Lines (PAL) standard, which is widely used in Europe. There are several key differences between NTSC and PAL in the way how the chroma portion of the composite signal is encoded. In NTSC, the chroma portion of a composite signal shifts in phase by one hundred eighty degrees between corresponding pixels on consecutive lines and frames. In PAL, the chroma component of the composite signal shifts in phase by approximately 270 degrees under the same condition.
A number of functions in a video display rely on the accurate detection of motion within the signal, such as noise reduction and deinterlacing. Another application of motion detection is the separation of luma and chroma information within a signal. In some video transmission standards, chroma information within a composite video signal can be carried within the same frequency band as luma information. The chrominance signal is separated out of the composite video signal and decoded, along with the normal intensity signal, to determine the color content of the input signal. A problem can arise when portions of the luma signal intrude into or near a frequency band associated with a chroma subcarrier. With 2-D comb filter, a portion of the intruding portion of the luma signal can be demodulated and output from the video decoder along with the chroma signal, resulting in a display of false color. Similarly, the chroma may also cause cross luma (e.g., dot crawls or hanging dots) in the luma decoded and output from the decoder. All the above mentioned artifacts are a result of incomplete separation of luma and chroma signals from the composite video.
The complete separation of luma and chroma, thus the elimination of false color and dot crawls can be achieved with better accuracy by comb filtering the signal across three dimensions (e.g., within a frame as well as successive frames), but this technique works and will be applied only if there is little or no motion in the video during this time. The correct application of 3-D comb filtering relies on accurate motion detection. Thus, by detecting the lack of motion across frames, the color artifacts within the composite video signal can be reduced by applying a 3-D comb filter. Unfortunately, in certain video formats such as PAL, motion detection is susceptible to color phase error. The color phase error can deceive a motion detection algorithm to mis-detect motion with a sequence of perfectly still images, degrading the quality of the video display.